1. Field of the Invention
The present invention relates to a magnetic disk apparatus which is capable of reducing the probability of the occurrence of read errors caused by scratches or the like on a surface of a magnetic disk, and which employs a different standard for determining a defect when registering a defective sector during a disk inspection from a standard used for regular reproduction.
2. Description of the Related Art
In some types of flexible disks adapted for high-density recording, a sector in a recording track area thereof is provided with a servo field. A signal in the servo field is used for servo control when making a magnetic head seek a target track and for servo control during on-tracking.
Furthermore, a data field is provided, following the servo field in one sector. SYNC, a servo address mark (SAM), and servo information, namely, A burst and B burst, are recorded in the servo field. Synchronization is performed using the SYNC, positional information regarding the sector is read using the SAM, and a positional correction of the magnetic head is made using the A burst and the B burst.
The data field is further provided with a SYNC area and a data area (DATA). Following synchronization accomplished by the SYNC, a data body is read from the data area. In this case, settings are made such that a read gate of a reading circuit is opened in a predetermined time on the basis of detection of a reference signal (STM) at the beginning of the SAM or in the SAM. The read gate is opened when a head is positioned in the SYNC area of the data field. As a result, while the read gate is open, a signal of the data body is read from the data area, following the SYNC.
This type of a magnetic disk for high-density recording is subjected to writing and reading data for inspection during a quality inspection following a manufacturing process. In the quality inspection, a sector from which data cannot be properly read is determined to be a defective sector, and the number of the sector is registered as the number of a defective sector in a maintenance area of the magnetic disk. When the magnetic disk is loaded in a regular disk apparatus, the maintenance area is first read, the number of the defective sector is recognized and control is carried out so that the defective sector will not be used for recording.
However, when a flexible magnetic head for high-density recording is loaded in a disk apparatus to perform recording or reproduction, there are cases wherein a recording error or a reproduction error occurs in a sector that has not been determined to be defective in the quality inspection. These errors are attributable to several causes including scratches on a disk surface incurred during repeated use of a magnetic disk since the magnetic disk is repeatedly loaded in a disk apparatus, variations in quality of an apparatus used for recording or reproduction, or changeable factors of operating environments, such as temperature and humidity, of magnetic disks.
As a result, it sometimes happens that no ECC error takes place in a sector during the quality inspection, so that the sector is judged to be nondefective, whereas a read error occurs during actual reproduction due to a scratch mentioned above even in a field that has not been registered as the defective sector. For example, if the standard signal (STM) in a servo field fails to be read due to a scratch or the like, then a read gate fails to open in a predetermined time based on the reference signal. This leads to a problem in that normal data of a low probability of occurrence of an ECC error fails to be read.
Accordingly, the present invention has been made with a view toward solving the problem described above, and it is an object thereof to provide a magnetic disk apparatus capable of reading normal data following the occurrence of a read error of a reference signal (STM) in, for example, a servo field, the read error being attributable to a scratch or the like on a disk surface.
It is another object of the present invention to provide a magnetic disk apparatus that has a strict standard set for a read error of the STM in a quality inspection prior to the shipment of disks so as to make it possible to recognize a sector that incurs such a read error as a defective sector.
To these ends, according to one aspect of the present invention, there is provided a magnetic disk apparatus having a head for reading recorded information from a disk provided with a servo field and a SYNC area and a data area following the servo field, a gate for applying a signal read by the head to a signal processing unit, and a control unit for controlling the gate; wherein the control unit carries out control such that the gate is opened after a set period of time (T1) based on a time at which a reference signal (STM) in the servo field has been detected from the signal read by the head, or the gate is opened after the elapse of a time corresponding to the set period of time (T1) from a moment at which the reference signal (STM) is supposed to be detected if the reference signal (STM) cannot be detected after a predetermined period of time (T) following the detection of a preceding reference signal (STM).
For instance, the control unit determines a difference (T1xe2x88x92Tw=T2) between a period of time (Tw) further elapsed from the predetermined period of time (T) and the set period of time (T1), and conducts control to open the gate in the determined T2.
The present invention can be applied to a case wherein a read error of the STM in a servo field takes place due to a scratch or the like on a disk surface when an attempt is made to read, using a regular disk apparatus, data from an area which has incurred no ECC error during the quality inspection of the disk and presents a low ECC error rate, and which accordingly has not been registered as a defective sector in the maintenance area of the disk.
If a read error of the reference signal occurs, then the control unit determines how much time (Tw) has further passed after the predetermined time (T) passed from the preceding reference signal. The control unit then calculates the period of time obtained by subtracting the elapsed time (Tw) from the set period of time (T1), namely, (T1xe2x88x92Tw=T2). If the reference signal read error occurs, the control unit implements processing to open the read gate after the period of time (T2) passes. This enables a data body of the data area to be read.
Alternatively, if an STM read error happens after the predetermined period of time (T) has passed from a preceding reference signal, the control unit may continue measurement of time after the predetermined period of time (T) has passed, adds the set period of time (T1) after the predetermined period of time has passed, and opens the gate when a time obtained by (T+T1) has passed.
In a preferred form of the present invention, a magnetic disk is provided with a maintenance area in which a defective sector is registered during a disk inspection, and if the STM is not detected within the predetermined period of time (T), then a sector from which the STM has not been detected is registered as a defective sector in the maintenance area during the disk inspection.
In a further preferred form, the processing for opening the read gate by setting the time difference (T1xe2x88x92Tw=T2) is performed only in a regular disk apparatus in which a disk after shipment is loaded, and the time difference is not set in a quality inspection prior to the shipment of the magnetic disk. In the quality inspection before shipment, a sector in which the reference signal read error has occurred is recorded as a defective sector in a maintenance field.
Thus, by providing more strict processing for dealing with the reference signal read error during the quality inspection prior to shipment than for the control processing in the regular disk apparatus makes it possible to achieve a lower probability of the occurrence of read errors in a disk loaded in the regular disk apparatus.